Merge pull request neetcode-gh#2225 from seinlin/15

Add 0015-3sum.c

Author: tahsintunan

c/0015-3sum.c

@@ -0,0 +1,72 @@
+int compareInt(const void *a, const void *b) {
+    return *(int*)a - *(int*)b;
+}
+
+/**
+ * Return an array of arrays of size *returnSize.
+ * The sizes of the arrays are returned as *returnColumnSizes array.
+ * Note: Both returned array and *columnSizes array must be malloced, assume caller calls free().
+ */
+int** threeSum(int* nums, int numsSize, int* returnSize, int** returnColumnSizes) {
+    int cap = 32;
+    int **result = (int**)malloc(sizeof(int*) * cap);
+    int *cols = (int*)malloc(sizeof(int) * cap);
+    int *triplet;
+    int sum, target, count;
+    int i, left, right;
+
+    qsort(nums, numsSize, sizeof(int), compareInt);
+
+    count = 0;
+    for (i = 0; i < numsSize - 2; i++) {
+        // The array is sorted, there is no possible tripet when this happen.
+        if (nums[i] > 0) {
+            break;
+        }
+        // skip element with same value to avoid duplicate triplets.
+        if (i > 0 && nums[i] == nums[i - 1]) {
+            continue;
+        }
+        target = 0 - nums[i];
+        left = i + 1;
+        right = numsSize - 1;
+        while (left < right) {
+            sum = nums[left] + nums[right];
+            if (sum > target) {
+                right--;
+            } else if (sum < target) {
+                left++;
+            } else {
+                triplet = (int*)malloc(sizeof(int) * 3);
+                triplet[0] = nums[i];
+                triplet[1] = nums[left];
+                triplet[2] = nums[right];
+
+                result[count] = triplet;
+                cols[count] = 3;
+                count++;
+
+                if (count == cap) {
+                    cap *= 2;
+                    result = (int**)realloc(result, sizeof(int*) * cap);
+                    cols = (int*)realloc(cols, sizeof(int) * cap);
+                }
+
+                left++;
+                right--;
+
+                // skip element with same value to avoid duplicate triplets.
+                while(left < right && nums[left] == nums[left - 1]) {
+                    left++;
+                }
+                while(left < right && nums[right] == nums[right + 1]) {
+                    right--;
+                }
+            }
+        }
+    }
+    *returnSize = count;
+    *returnColumnSizes = cols;
+
+    return result;
+}